We did get a few days warning with nordic.
I recently updated the removed games list in the GOGwiki and went through the forum posts for each game removed, most of the times there was some kind of warning. If a 30 minute warning counts or not is another question enterely...
Full-on removals, yes, there would likely be a heads-up, though not necessarily a long one.
But when it comes to replacing games with censored versions, I'm thinking of the more subtle changes here such as when content is dropped and we only find out because it shows up in MaGog's logs. I suspect replacing a version with a censored edition would fall into that category and GOG would make no announcement of the change, except possibly in the Update thread.